1. Field of the Invention
The present invention relates to an eddy current flaw detection sensor and method. In particular, the invention relates to arrangement of excitation and detection coils of an eddy current flow detection sensor in which the excitation coils are disposed on both sides of at least one detection coil and a method of detecting a circumferential crack occurring at a tubular test object by use of the sensor.
2. Description of the Related Art
The heat transfer tube of a heat exchanger installed in the cooling material purification system, etc. of an atomic power plant is subjected to periodic inspections to determine if a flaw such as a crack or the like occurs or not. An eddy current flaw detection method is usually adopted as a method of detecting a heat transfer tube of such a case, because this method exhibits high surface sensitivity, enables high-speed and noncontact flaw detection and additionally is appropriate for automatic flaw detection and remote control.
The eddy current flaw detection method is a nondestructive test method. In this method, alternating voltage is applied to an excitation coil provided for the eddy current flaw detection sensor to generate an eddy current in a planar direction of a test object made of a conductor while an eddy current flaw detection sensor scans the test surface. When an eddy current is caused to flow to a possible defective portion, a change of signal appearing in a detection coil is observed. Thus, the presence or absence, position, size and the like of a flaw is determined.
An eddy current flaw detection probe is proposed as the eddy current flaw detection sensor which has been applied to a tubular test object such as a heat transfer tube or the like. In the eddy current flaw detection prove shown in FIGS. 17A and 17B, two excitation coils 101, 102 reverse to each other in winding direction are arranged almost parallel to each other. Between the two excitation coils 101, 102, detection coils 103a to 103h having the same winding direction are disposed almost parallel to the axial direction of the excitation coils 101, 102 and radially around the axis thereof. See JP-A-2003-149210, [0013] to [0016].
This eddy current flaw detection probe performs flaw detection from the inner surface of the tubular test object A by being inserted into the test object A. More specifically, excitation current is applied to the excitation coils 101, 102 while the eddy current flaw detection probe is inserted in the tubular test object A. At that time, eddy currents flowing in the circumferential direction of the test object A are superposed on the wall surface of the test object A facing the intermediate area between the two excitation coils 101, 102. As shown in FIG. 17B, an eddy current distribution B is formed around the detection coils 103a to 103h so as to reach the deep layer portion of the test object A. Accordingly, the position, shape, sized and the like of a flaw C can be detected by the detection coils 103a to 103h detecting bypass eddy current D which flows in the axial direction X-X of the test object A while bypassing the flaw C extending parallel to the axial direction X-X of the test object A as shown in FIG. 18.
An eddy current probe described below has been proposed as the eddy current flaw detection sensor of this type. As shown in FIGS. 19A and 19B, this eddy current probe includes a columnar-formed probe main body 200 and two coil units arranged in the axial direction of the main body 200. Each of the coil units has a combination of a plurality of eddy current generation coils 201 and of detection coils 202. The eddy current generation coils 201 and the detection coils 202 constituting one of the coil units are arranged to be offset from those of the other coil unit in the circumferential direction of the probe main body 200. See JP No. 3406649, [0019] to [0023]. This patent document also discloses a technique of connecting in series the plurality of eddy current generation coils 201 constituting each coil unit.
The eddy current flaw detection sensor described above can detect a flaw occurring in the axial and circumferential directions of a tube.